The invention relates to a pulse radar apparatus provided with a transmitting and receiving unit, whereby the received target echoes are sampled and digitised after detection. The apparatus includes an n-point DFT processing unit; a threshold circuit; and a cluster combiner circuit for combining radar cells, collectively covering one single target, to form a cluster from the signals obtained from the threshold circuit. The apparatus produces a signal representing the target covered by the cluster and hence the target range and azimuth.
Such a pulse radar apparatus is known from the European Pat. No. EP-A-0.057.949, (corresponding to U.S. Pat. No. 4,524,360) describing the presence of a cell classification unit between the threshold circuit and the cluster combiner circuit. This unit facilitates a preliminary distinction between first- and multiple-time-around echoes in the cells on account of the spectrum range, as here the first-time-around echoes, but the multiple-time-around echoes are not coherently detected. A final distinction between first- and multiple-time-around echoes is made in the cluster. This system is unsuitable in case of coherent or non-coherent detection of both the first- and the multiple-time-around echoes, the reason being that in such a case the spectrum range no longer shows any difference. In the situation where a target-representing signal is derived from a first- or a multiple-time-around echo, reference is hereinafter made to a first- or a multiple-time-around target.
The present invention has for its object to provide, irrespective of the fact of coherent or non-coherent detection of the first- or multiple-time-around echoes, means for determining whether the target-representing signal is derived from a first- or a multiple-time-around echo. The invention is charcterised in that: the pulse repetition period of the radar apparatus contains m wobbles; the cluster combiner circuit comprises means for determining the magnitude of the local maximum values present in the DFT spectrum covered by the target-representing signal as well as an indication which of the DFT-processor output channels produces the maximum values; and a classification unit connected to the cluster combiner circuit is incorporated for determining a target to be a multiple-time-around target. This determination is made subject to the conditions that: for the target-representing signals produced by the cluster combiner circuit the difference in the cluster-defined target ranges correspond or substantially correspond with a particular wobble period; the difference in aximuth of the clusters is within predefined narrow limits; the magnitudes of the local maximum values of the DFT spectra are equal or substantially equal to each other; and the local maximum values are present in either the same DFT-processor output channel or output channels differing in frequency from each other over a value of (k/m)n times the bandwidth of the output channels concerned, where k=1, 2, . . . , m-1.
It is also possible to extend the pulse radar apparatus according to the invention in a simple way in order that, if the classification unit has not defined a target as being a multiple-time-around target, this unit establishes the value of P from the ratio between the local maximum values, where 2.sup.P is equal to the quotient of the doppler frequency and the pulse repetition frequency.